Enzymes are proteins that catalyze biochemical reactions without being consumed in the process. They can catalyze both catabolic reactions that break down molecules, and anabolic reactions that build molecules up. Enzymes have an active site that binds to specific substrate molecules and a regulatory region that can be influenced by cofactors, coenzymes, or inhibitors. The binding of a substrate to an enzyme's active site is described by either the lock-and-key or induced-fit models. Factors like pH, temperature, enzyme and substrate concentrations can affect an enzyme's activity level.